Nature of ore-forming fluid and formation conditions of BIF-hosted gold mineralization in the Archean Amalia Greenstone Belt,South Africa: Constraints from fluid inclusion and stable isotope studies |
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| Institution: | 1. Department of Geology, Andhra University, Visakhapatnam 530003, India;2. National Geophysical Research Institute (Council of Scientific and Industrial Research), Uppal Road, Hyderabad 500007, India;3. National Institute of Oceanography (Council of Scientific and Industrial Research), Regional Centre, Visakhapatnam 530017, India;4. Department of Geology, Central University of Karnataka, Gulbarga, India;5. School of Earth Science and Resources, China University of Geosciences, Beijing, China;1. Guangzhou College South China University of Technology, Guangzhou 510800, China;2. Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China;3. Beijing Institute of Geology for Mineral Resources, Beijing 100012, China;4. Centre for Exploration Targeting, ARC Centre of Excellence for Core to Crust Fluid Systems, The University of Western Australia, Crawley, WA 6009, Australia;5. MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, CAGS, Beijing 100037, China;6. Henan Academy of Land and Resources Sciences, Zhengzhou 450053, China;7. Department of Earth Sciences, University of New Brunswick, N.B, E3B 5A3, Canada |
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| Abstract: | Orogenic gold mineralization in the Amalia greenstone belt is hosted by oxide facies banded iron-formation (BIF). Hydrothermal alteration of the BIF layers is characterized by chloritization, carbonatization, hematization and pyritization, and quartz-carbonate veins that cut across the layers. The alteration mineral assemblages consist of ankerite-ferroan dolomite minerals, siderite, chlorite, hematite, pyrite and subordinate amounts of arsenopyrite and chalcopyrite. Information on the physico-chemical properties of the ore-forming fluids and ambient conditions that promoted gold mineralization at Amalia were deduced from sulfur, oxygen and carbon isotopic ratios, and fluid inclusions from quartz-carbonate samples associated with the gold mineralization.Microthermometric and laser Raman analyses indicated that the ore-forming fluid was composed of low salinity H2O-CO2 composition (~3 wt% NaCl equiv.). The combination of microthermometric data and arsenopyrite-pyrite geothermometry suggest that quartz-carbonate vein formation, gold mineralization and associated alteration of the proximal BIF wall rock occurred at temperature-pressure conditions of 300 ± 30 °C and ～2 kbar. Thermodynamic calculations at 300 °C suggest an increase in fO2 (10?32–10?30 bars) and corresponding decrease in total sulfur concentration (0.002–0.001 m) that overlapped the pyrite-hematite-magnetite boundary during gold mineralization. Although hematite in the alteration assemblage indicate oxidizing conditions at the deposit site, the calculated low fO2 values are consistent with previously determined high Fe/Fe + Mg ratios (>0.7) in associated chlorite, absence of sulfates and restricted positive δ34S values in associated pyrite. Based on the fluid composition, metal association and physico-chemical conditions reported in the current study, it is confirmed that gold in the Amalia fluid was transported as reduced bisulfide complexes (e.g., Au(HS)2?). At Amalia, gold deposition was most likely a combined effect of increase in fO2 corresponding to the magnetite-hematite buffer, and reduction in total sulfur contents due to sulfide precipitation during progressive fluid-rock interaction.The epigenetic features coupled with the isotopic compositions of the ore-forming fluid (δ34SΣS = +1.8 to +2.3‰, δ18OH2O = +6.6 to +7.9‰, and δ13CΣC = ?6.0 to ?7.7‰ at 300–330 °C) are consistent with an externally deep-sourced fluid of igneous signature or/and prograde metamorphism of mantle-derived rocks. |
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| Keywords: | South Africa Amalia greenstone belt Gold Banded iron formation Redox condition Fluid inclusions Carbon-oxygen isotopes |
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